Central diabetes insipidus physiology — FRCA Primary MCQ

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ModeratePhysiologyCentral diabetes insipidus physiologyFRCA Primary

A patient has central diabetes insipidus after pituitary surgery. Urine output is 400 mL h-1, serum sodium is 152 mmol L-1 and urine osmolality is low. Which hormone deficiency best explains the findings?

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Correct answer: D — Renin

Central diabetes insipidus results from deficient antidiuretic hormone, causing inability to concentrate urine, high free-water loss and hypernatraemia. Aldosterone deficiency causes salt wasting and hyperkalaemia rather than dilute polyuria with hypernatraemia. Renin is part of blood pressure and sodium regulation but does not directly set collecting duct water permeability. The pearl is that postoperative polyuria requires paired serum and urine osmolality interpretation.

Reference: RCoA Primary FRCA Syllabus 2026; Ganong's Review of Medical Physiology; West's Respiratory Physiology